1. Field of the Invention
The present invention relates to a wiring substrate for mounting semiconductor elements and a fabricating method thereof.
2. Description of Related Art
As a wiring substrate for mounting semiconductor elements, which an LSI (Large Scale Integrated) chip is mounted, which is connected to a printed wiring, etc., and which is called, for example, an interposer for mounting semiconductor elements, there are two kinds of wiring substrates: one with a resin base such as a polyimide film, and the other with a metal base such as copper. Here, a description will be given on a fabricating method of the wiring substrate with a polyimide film base.
A polyimide film is prepared as a base, and a thin copper film having a thickness of about, for example, 0.2 xcexcm is formed on each main surface of the base by, for example, sputtering. Next formed by drill machining or press working is a through hole for connecting to one another wiring films that are to be formed on those main surfaces. Thereafter, a copper film with a thickness of about, for example, 5 xcexcm is formed by electroless plating on the surface of each of the above thin copper films. A resist film for patterning is further formed on this copper film placed on each surface of the base, and a copper wiring film (with a thickness of 30 xcexcm, for example) is formed on each of the copper film by electroplating while using the resist films as masks. With this, one of the copper wiring films, which is the one formed on the front side of the base constitutes a normal circuit wiring and the other one formed on the back side of the base constitutes a ground line and a power source, line.
After removing the resist films used as masks, then removed by soft etching of copper are the thin copper films that have been formed over the entire surfaces as under films upon formation of the copper wiring films prior to the formation of the above resist films. By this removal, the copper wiring films are no longer electrically short-circuited with the thin copper films, resulting in copper wiring films that are independent of each other.
An insulating resin film is next applied to the front side surface and is patterned through exposure and development so that openings are formed in a portion where a solder ball is to be formed and a portion to be joined with an LSI. After that, the base is selectively etched from its back side to expose a portion of the copper wiring film, which is to be joined with the LSI, to enhance by, for example, gold electroless plating the connectivity of the copper wiring film surface to the ball electrode such as the solder ball or to the LSI. Thus fabricated is the wiring substrate called the interposer.
The above wiring substrate is bonded at its back surface side to an LSI chip through a buffer adhesive, and then lead ends of the copper wiring film are micro-joined with electrodes of the LSI chip. After sealing the micro-joined portion with a resin, one of the copper wiring films, which to on the opposite side of the base to the LSI chip, is plated, and the solder ball is mounted on a portion exposed to the opening of the insulating resin film. The solder ball is then shaped by reflowing.
Subsequently, a description will be made on a method of fabricating a conventional wiring substrate of the type with a metal base, such as copper. A base made of, for example, copper and having a thickness of about, for example, 100 to 200 xcexcm is prepared. A resist film is formed so as to have a pattern negative to the pattern of a copper wiring film to be formed. Using this resist film as a mask, a thin gold film is formed by electroplating on one surface of the copper base, and the wiring film is then formed by electroplating of the copper film. On a region where this wiring film is formed, an insulating film having an opening in a portion to form a ball electrode and having a pattern that does not cover a portion of the wiring film, which is used as electrodes of an LSI chip, is formed. After the ball electrode is formed from, for example, nickel or gold by plating in the opening portion of this insulating film, the base, except for its periphery, is removed by selective etching from the back surface side to expose the back surface side of the wiring film. Thus, the wiring substrate called the interposer is fabricated. This wiring substrate is bonded at its back surface side to the LSI chip through a buffer adhesive, leads of the wiring film are connected to electrodes of the LSI chip, and the substrate is sealed with resin, thereby completing the mounting of the LSI chip.
Wiring substrates of the type with a polyimide film base generally have problems as follows. First, when circuits are formed on both surfaces of the base and are connected to each other to obtain a two-layer circuit with the aim of high integration, mechanical punching out is required. The hole punched out has to be minute as high integration and to downsizing of the substrate are taken, which is likely to make it difficult to connect to one another the circuits in the two-layer circuit. In addition, processing accuracy (about position and shape) of the hole, a final package outer shape and ball position accuracy are not easily be enhanced, and hence it is becoming more and more difficult to achieve the accuracy demanded.
Also, having the base of a polyimide film, the wiring substrate is hard to have sufficiently enhanced physical strength upon completion as a circuit board. Therefore, it can not avoid a problem of high likelihood of deformation, etc., at the packaging process.
Further, since the polyimide film forming the base is an insulating material and it is difficult to apply electric potential, it makes electroplating virtually impossible. This brings about another problem of the solder ball being required to be mounted in post-attachment at the packaging process. Namely, because of extremely poor adherence related to the joining strength between solder and copper, a very large area has to be saved for the solder ball mounting portion in the wiring film in order to prevent a defeat of ball falling off. This leads to a reduced number of wirings that are allowed to thread through (be formed between) the solder balls adjacent to one another in the arrangement pitch of the solder balls, which gives rise to still another problem by being a great cause for blocking high integration.
In order to improve reliability against falling off of the ball, which is judged in the temperature cycle test performed after the balls are mounted to the substrate, an area allotted for every solder ball needs to be large. For that reason, the number of wirings running through (formed between) the balls is restricted, presenting still another problem of putting limitation on design in which densification is intended by increasing the number of the balls.
The polyimide film forming the base also serves as a carrier in the fabrication. The film is thus not easy to form thinner, making it hard to lower the usage amount and cost of the material, and further brings about a problem of poor connectivity between the circuits on both surfaces. Still further, the thick polyimide film absorbs a lot of moisture, causing the package crack at the time of mounting.
Therefore, a wiring substrate using as the base a metal instead of the polyimide film has been developed. The wiring substrate as such does solve a portion of the above-described problems inherent in the wiring substrate using the polyimide film as the base, but that conventional wiring substrate is not yet free from problems. That is, since the base to formed from a metal, in order to form a circuit on its surface by electroplating, it is required to remove most of the metal forming its base by selective etching from the back surface. The substrate therefore should take a considerably complicated ma structure in the case of forming the two-layer wiring circuit. Further, when the whole base under the wiring film is removed, the substrate is weak to the deformation from the viewpoint of film strength, and is difficult to mount.
In addition, enhancement of mounting density is demanded for the wiring substrates described above. This mounting density may be remarkably enhanced if the LSI chip is mounted on the main surface on each side of the wiring substrate, or if a plurality of wiring substrates with the LSI chips mounted thereto are layered. However, that has not been an easily accomplished work in prior art.
The present invention has been made to solve those problems and, therefore, an object of the present invention is to strengthen the rigidity of a wiring substrate for mounting semiconductor elements, to prevent shrinkage due to temperature shift based on the difference in coefficient of thermal expansion between a wiring film and an insulating film made of a resin, to enhance, by making it possible to form terminals such as the wiring film and a ball electrode through electroplating, its film quality and stability and, further, to connect a base with the wiring film so that the base may be utilized as a ground line, a power source line, etc.
Another object of the present invention is to provide a wiring substrate for mounting semiconductor elements to which an LSI chip may be mounted on the main surface of each side, and to make it possible to layer on one another a plurality of wiring substrates for mounting semiconductor elements each having the LSI chip mounted thereto.
According to the present invention, there is provided a wiring substrate comprising: a base made of a metal; and at least one layer wiring formed on the base through an insulating film, the layer wiring having a wiring film formed by electroplating, wherein the base is selectively etched.
According to the present invention, there is provided a method for fabricating a wiring substrate comprising the steps of: forming at least one layer wiring on a base made of a metal through an insulating film, the layer wiring having a wiring film formed by electroplating; and selectively etching the base.
According to the present invention, there is provided a wiring substrate comprising: a base made of a metal; a first insulating film having openings formed on the base; at least one layer wiring formed on the first insulating film, the layer wiring having a wiring film made of a metallized film at a lower portion; and a second insulating film formed on a region that the layer wiring is formed, except for a portion, wherein the base is selectively etched to partially expose a back surface of the wiring film.
In the wiring substrate of the present invention, the base is partially removed to form at least one selected from a group having a ground layer, a power source plane, a terminal, a dam and a reinforcement portion, and the opening formed in the first insulating film is filled with the wiring film to connect the wiring film to at least one selected form the group.
In the wiring substrate of the present invention, the metallized film is made of a wiring film material and a material having selective etching property.
In the wiring substrate of the present invention, the partially removed base forms a terminal for connecting with other member.
In the wiring substrate of the present invention, a terminal to formed on a portion of the region that the layer wiring is formed, by partially removing the second insulating film, and a hole for filling a buffer is formed at a position corresponding to the terminal of the base, the buffer is filled in the hole.
In the wiring substrate of the present invention, on the region that the layer wiring is formed, a portion where the second insulating film is partially removed serve as an opening for forming a bump electrode at which the layer wiring is partially exposed, and the bump electrode to be connected to the LSI chip by flip chip bonding are formed in the opening.
In the wiring substrate of the present invention, on the region that the layer wiring is formed, a portion where the second insulating film is partially removed serve as an opening for forming a bump electrode at which the layer wiring is partially exposed, and the bump electrode to be connected to other member are formed in the opening.
According to the present invention, there is provided a wiring substrate comprising: at least one layer wiring film formed on one side of the resin film having openings; and two kinds of metal bumps which are formed on the other side of the resin film, which are connected to the layer wiring film through the openings and which are different in height from each other.
In the wiring substrate of the present invention, lower metal bumps are bumps for flip chip bonding, and an LSI chip is bonded to the lower metal bumps.
In the wiring substrate of the present invention, an chip is disposed on the one side of the resin film where the layer wiring films are formed.
According to the present invention, there is provided a method for fabricating a wiring substrate comprising the steps of: selectively forming first solder films on one main surface of a base metal; forming a metal film on the one main surface of the base metal including the first solder films; forming an insulating film having openings on the metal film at positions corresponding to metal bumps to be formed later; forming at least one layer wiring on the insulating film; forming second solder films on the other main surface of the base m at positions where higher metal bumps are to be formed; and etching the base metal from the other main surface side using the second solder films as masks, and etching the metal film using as masks the first solder films and the second solder films, thereby forming higher metal bumps made from the metal film and the base metal and lower metal bumps made from the metal film.
In the fabricating method of the present invention, reflowing treatment is applied to the first and second solder films after forming the higher metal bumps and the lower metal bumps, so that the higher metal bumps and the lower metal bumps are covered with solders of the first and second solder films.